Raman Investigation of the 2D Ferromagnet AgVP₂Se₆

Many metal thio/seleno-phosphates (MTPs) are 2D magnets that exhibit diverse optoelectronic and multiferroic properties. While ternary MTPs typically display antiferromagnetism (AFM) and ferroelectricity, quaternary MTPs enable controlled tuning of ferroic behaviors by introducing different metal species to the lattice. In AgVP₂Se₆_, incorporation of Ag transforms the AFM state of VPSe₃ to a ferromagnetic (FM) phase with a Curie temperature of 18 K [1].  The low structural symmetry of AgVP₂Se₆ produces a rich Raman spectrum, but prior studies have reported inconsistent mode assignments [1,2] and lacked low-temperature measurements across the FM transition to assess spin-phonon coupling. We address these gaps through temperature-, polarization-, and magnetic field-dependent Raman spectroscopy of AgVP₂Se₆ crystals grown by chemical vapor transport (CVT) and flux methods. We observe 23 Raman-active modes in both compounds; all are accounted for by density functional theory calculations. We attribute subtle differences in mode intensity and pattern between CVT and flux-grown samples to the increased stacking disorder in the latter. Polarization measurements reveal the symmetry (A or B) of each phonon mode, and angle-resolved spectra yield complex-valued Raman tensors for major modes. Temperature-dependent spectra show no discernible changes through the FM transition but do show large changes near 100 K, suggesting a previously unreported structural phase transition. These results establish a foundation for future studies of magnon excitations and magnetoelastic interactions in AgVP₂Se₆.